If you're assuming that they won't be punished if they convinced the other person, then that's true. That would be a conflict of interest and hint at them starting with the bottom line.

If you don't assume that, then it sounds like ad hominem combined with circular logic. Them being a murderer doesn't mean their argument is wrong. In fact, since they're living the conclusion, it's evidence that they actually believe it, and thus that it's write. Furthermore, them being a murderer is only bad if you already accept the conclusion that it's not OK to kill the other person and take their stuff.